To work out yarn substitutions,multiply the number of balls of the original yarn by the length per ball,then divide this number by the length per ball of the substitute yarn which will give you the number of balls for the substitute yarn ( length per ball is approximate,probably best to round up rather than down especially on one colour items,or add one extra ball ). If the length is approximately the same for both yarns you can probably miss working this out,but differences of 10 metres or more it is worth doing. Tensions are as given on the ball bands,you may need to change your knitting pin size to match the tension.